With Donald Trump getting more TV coverage than Charlie Sheen and rising in the polls among Republicans, it is not a surprise that the knives have come out for him. “He’s just another liberal,” screams the libertarian Club for Growth. “He’s not one of us,” echoes Karl Rove.

All of that may be true, but one piece of cited evidence is quite puzzling. Mr. Trump’s GOP opponents accuse him of wanting to get tough on China and of being a protectionist. Since when does that mean one is not a conservative? For most of its 157-year history, the Republican Party has been the party of building domestic industry by using trade policy to promote U.S. exports and fend off unfairly traded imports. American conservatives have had that view for even longer.

At the beginning of this nation, Alexander Hamilton and his followers were staunch conservatives who helped found American capitalism - and avowed protectionists. By contrast, Thomas Jefferson - the founder of the Democratic Party - was much more of a free trader. During the first half of the 19th century, pro-business politicians like Henry Clay were ardent supporters of an “American system” that would use tariffs to promote American industry. Clay’s political descendents - such as Abraham Lincoln - went on to form the Republican Party. Every Republican president starting with Lincoln - and for almost 100 years thereafter - generally supported tariffs, while Democrats tended to promote free trade.

Would anyone argue that presidents like William McKinley, William Howard Taft and Calvin Coolidge were not conservatives - or that free traders like Woodrow Wilson and Franklin D. Roosevelt were not liberals?

Skepticism toward pure free-trade dogma can be seen as well in more recent Republican leaders. The icon of modern conservatism, Ronald Reagan, imposed quotas on imported steel, protected Harley-Davidson from Japanese competition, restrained import of semiconductors and automobiles, and took myriad similar steps to keep American industry strong. The same can be said of Richard Nixon. In 1971, Nixon imposed a temporary tariff on all imports in response to what he perceived to be unfair foreign economic policies. No one would accuse Nixon of being a “liberal” - but his approach was in some ways even more trade-restrictive than what Mr. Trump has suggested.

In light of these facts, can anyone really think that getting tough with China is a “liberal” idea? Do you think that any of the conservatives and Republicans listed above would allow a foreign adversary to use currency manipulation, subsidies, theft of intellectual property and dozens of other forms of state-sponsored, government-organized unfair trade to run up a more than $270 billion trade surplus with us and to take U.S. jobs?

On a purely intellectual level, how does allowing China to constantly rig trade in its favor advance the core conservative goal of making markets more efficient? Markets do not run better when manufacturing shifts to China largely because of the actions of its government. Nor do they become more efficient when Chinese companies are given special privileges in global markets, while American companies must struggle to compete with unfairly traded goods.

Many modern conservatives have already figured this out. Only last September, the House of Representatives voted to give the president expanded authority to impose tariffs on virtually all Chinese imports in response to China’s policy of keeping its currency at an artificially low value. Republican House members voted 99 to 74 in favor of this legislation. Recent polling data show that rank-and-file conservatives are even more skeptical about free trade. Last September, an NBC/Wall Street Journal poll showed that 61 percent of Tea Party sympathizers think that free trade has hurt the United States. A November Pew Research poll found that 52 percent of Republicans think that increased trade with China is bad for the United States and that only 28 percent of Republicans think free-trade agreements are good for the United States.

When viewed in this context, the recent blind faith some Republicans have shown toward free trade actually represents more of an aberration than a hallmark of true American conservatism. It’s an anomaly that may well demand re-examination in the context of critically important questions facing all conservatives on trade policy.

Given the current financial crisis and the widespread belief that the 21st century will belong to China, is free trade really making global markets more efficient? Is it promoting our values and making America stronger? Or is it simply strengthening our adversaries and creating a world where countries who abuse the system - such as China - are on the road to economic and military dominance? If Mr. Trump’s potential campaign does nothing more than force a real debate on those questions, it will have done a service to both the Republican Party and the country.

Robert E. Lighthizer was a deputy U.S. trade representative in the Reagan administration and is now an international trade lawyer in Washington.